One of the conventional methods of detergent bar manufacture is casting. In the casting process, a detergent composition in a heated, mobile and readily pourable state is introduced into the top of an enclosed cavity of the desired shape within a mould and the temperature of the composition is reduced until it solidifies either totally or partially, such that the bar is handleable. The mould is generally two-part, and the bar can be removed by opening the mould.
In order to be castable, the detergent formulation must be mobile and readily pourable at the elevated temperatures employed. The detergent melt, once in the mould, tends to cool slowly and unevenly. This can lead to unwanted structural orientations and segregation of ingredients.
Detergent formulations may be desired to be formed into bars in such a condition that they are not sufficiently fluid to be cast by conventional means. In this case, pressure can be used to deliver the detergent composition to the mould in, for example, an injection moulding process. The detergent composition will still require some further cooling and solidification within the mould cavity.
A major problem with such processes is that detergent compositions shrink in the mould as they cool and solidify. This is highly undesirable as the mould cavity is intended to impart a distinctive shape on the bar and often also a logo of some kind. Shrinkage can result in dimples, wrinkles or voids in the bar, or a depression at the fill point. Which of these bar imperfections occurs depends on factors such as the nature of the detergent composition at fill, the cooling mechanism employed, and the form and surface of the mould.
A further problem particularly associated with the delivery of semi-solid detergent compositions to a mould, is the formation of "weld lines" in the bars. Without being limited by theory, it is believed that as a material is delivered to a mould cavity, flow fronts of the material are created in the cavity as material is added and the cavity fills. Weld lines in the final bars are a result of interfaces between flow fronts of detergent material inside the mould cavity which have not blended together. Such weld lines may be visible to the consumer, and can lead to weaknesses in the bar, which may in turn lead to cracking in use.
Therefore, there is a need for a process for forming detergent compositions into good quality bars which have good appearance and physical characteristics. Such a process should prevent the imperfections so far associated with shrinkage, and, should allow the detergent composition to be fed to the mould in any desired physical state, and the bar formed to be any desired shape, with a well-defined logo if required.
The present inventors have found that if a pressure is applied to a detergent composition contained within a mould cavity then the problems associated with shrinkage of the detergent composition as it cools can be reduced.
Furthermore, the problems encountered when forming bars from semi-solid material, e.g. weld lines, can be alleviated and better logo definition can be obtained.